Fluorozirconate and fluorohafnate glasses are unique nonoxide materials which include zirconium tetrafluoride and hafnium tetrafluoride, respectively, as major constituents. These multicomponent glasses are commonly referred to as the heavy-metal fluoride glasses.
The heavy-metal fluoride glasses have been found to have certain desirable physical and chemical characteristics which make them ideally suited for a wide variety of applications in optical systems. Heavy-metal fluoride glasses are prime candidates for use as optical-fibers for communications or transmission of optical power. A survey of the development of heavy-metal fluoride glasses, their properties and their uses is set forth by Martin G. Drexhage in Chapter 4 of the Treatise on Materials Science and Technology, Vol. 26 (1985) pages 151-243. The contents of this chapter are hereby incorporated by reference.
Millions of dollars are being spent each year to develop heavy-metal fluoride glasses specifically for IR fiber optic applications. The successful utilization of such fibers hinges on high optical quality in the IR range and superior mechanical properties of the fluorozirconate or fluorohafnate glass.
In order to achieve good optical transmission (less than 0.1 dB/km absorption) in the near-IR (2-5 micrometer) through optical fibers made from zirconium or hafnium tetrafluoride based glasses, the transition-ion content (Fe.sup.+2, Cu.sup.+2, Ni.sup.+2, and Cr.sup.+3) must be in the parts per billion (ppb) range. The anion impurity level (OH.sup.- and O.sup.-2) must also be correspondingly low in order to achieve desired IR transmission characteristics.
Commercially available zirconium and hafnium tetrafluorides generally have cation anion and carbon contaminant levels which are unacceptably high. Accordingly, it is necessary to purify these two metal tetrafluoride commercial materials prior to their use in fabrication of the various heavy-metal fluoride glasses. We have had success in the past at reducing the anion concentration in commercial materials to acceptable levels by reactive atmosphere processing (RAP) utilizing carbon tetrachloride and hydrogen fluoride. See U.S. Pat. No. 4,341,873 issued on July 27, 1982, which is assigned to the same assignee as the present application.
Removal of the cation impurities present in commercially available zirconium and hafnium tetrafluorides has proved to be more difficult. This has been especially true for the removal of Fe impurities.
Sublimation and distillation are separation techniques which have been used to purify heavy-metal halides, such as zirconium and hafnium tetrafluorides. These separation techniques are for the most part satisfactory for removing the majority of cation impurities found in commercially received material, such as the alkaline earth and 4f impurities. However, sublimation and/or distillation have been only partially effective in removing Fe impurities due to the relatively high vapor pressure of Fe.sup.+3. During sublimation or distillation of the zirconium or hafnium tetrafluoride, the concentration of the impurities increases in the remaining solid or liquid material so that vaporization of the Fe.sup.+3 begins to occur along with the zirconium or hafnium tetrafluoride to thereby contaminate the sublimate. In order to prevent iron from contaminating the sublimate or distillate, it is necessary to stop the sublimation or distillation after only part of the material has been vaporized. This is undesirable, of course, since the residue must be discarded even though a high percentage of the metal tetrafluoride remains in the residue. As a result, a substantial waste of material occurs. Further, the degree of purity obtainable by sublimation or distillation alone does not reduce the Fe content into the ppb range, i.e., below 1 ppm.
In view of the above, it would be desirable to provide an improved sublimation or distillation method which includes all the advantages of these two simple separation procedures, and also increases the efficiency of the methods by allowing removal of Fe impurities to ppb levels in a one step procedure.